For example, an optical disk is constituted such that a recorded surface of a substrate made of synthetic resin such as polycarbonate is provided with a reflective coat which is formed by vapor deposition a metal such as gold or aluminum onto the recorded surface, and a protective coat such as ultraviolet cured resin on the reflective coat for avoiding oxidation of the same. In this optical disk, the recorded surface of the disk substrate is formed with concave-convex pits for recording data, and is provided with the reflective coat thereon.
In such an optical disk, it is required to recognize the recorded contents, from the exterior appearance. Thus, it is generally performed to offset print or screen print a label or index onto the protective coat, by means of ultraviolet curable ink. However, in such a printing procedure, there is required a plate making process since the plate is to be prepared in advance, deteriorating the efficiency of printing in case of many kinds or a small amount.
Thus, there has been proposed to adopt an electrophotographic process to thereby omit plate making, such as in Japanese Unexamined Patent Publication No.5-212857.
Generally, in an electrophotographic process, the charged toner on a transfer medium such as drum or belt, is adsorbed onto the object by applying an electric charge having a polarity opposite to that of the toner from the side (back surface) opposite to the transfer medium of the matter to be printed while adhering the transfer medium onto the object, or the toner is promoted to separate from the transfer medium by supplying an electric charge having a polarity the same as that of the toner to the side opposite to the transfer medium of the matter to be printed.
Unfortunately, if it is intended to print an optical disk by supplying an electric charge from the side of the disk opposite to the transferring medium, as in the conventional electrophotographic process, there is caused such a problem that the toner is not assuredly transferred onto the disk, such as due to the thickness of the latter.
To avoid this problem, the aforementioned Japanese Unexamined Patent Publication No.5-212857 has made an endeavor to previously charge the surface of the optical disk, i.e., the side (surface) to be printed onto which the toner is to be transferred, with a polarity opposite to that of the toner, by bringing a charging scorotron close to such a surface, before the transfer of toner from the transfer medium. At the same time, it is intended to promote the separation of the toner from the transfer medium, by applying a high voltage having a polarity same with the toner from the back side of the transfer medium by a transfer corotoron or transfer roll.
However, there has remained such a problem that the printing lacks uniformity, and the printable area is limited, even by this technique.